Hmm, I think the yearly focus you see is coincidence. Most likely Nintendo just releases what is ready. A few patterns do emerge though:
1. I am pretty sure Nintendo planned about their big hitters Zelda, Mario and Smash. So they planned to release with Zelda, fill out the first year with Mario and the second with Smash. That probably was deliberate.
2. I always get the impression Gamefreak does whatever. As long as Pokemon sells Nintendo probably is willing to let them. Some spinoff titles from the Pokemon Company are probably more under Nintendos control, but the mainline Pokemons are Gamefreaks deal. So Pokemon is ready this year, because Gamefreak needed until now to get it ready. I have the feeling Gamefreak was testing some stuff with Let's Go.
3. Third party support need more time to ramp up than first party. Reluctant third parties that jumped on the ship after the Switch took off probably needed until now to get their stuff ported. And so we see more 3rd-party ports, also more ambitious stuff.
4. That you put WiiU ports in the second year is obviously wrong, they happened in 2017 too. WiiU ports are just cheap ways to get more money off the development of these titles that not many played on WiiU. They will be sprinkled in here and where, this continues this year (NSMBUD) and maybe even still next year.
The rest is just: we release it when it is ready. Metroid Prime probably was planned for this year, but needed to be scrapped. We know Animal Crossing got delayed. Bayonetta maybe was delayed, they never mentioned a date before so we don't know. I have a strong feeling Luigis Mansion probably should've been propping up last years lineup (which looked light in comparison). But development takes times, sometimes more than planned. So these games get released then they are ready.